Aftershocks from Britain’s stunning decision to leave the European Union continued Sunday, as the country’s politics and its relations with the world plunged into deeper uncertainty. U.S. Secretary of State John Kerry, already in Europe, added previously unannounced stops in London and Brussels to his trip at the last minute.

Kerry plans to meet with British Foreign Secretary Philip Hammond and other officials as part of a process of maintaining ties with Washington's top ally in a new era when Britain is less engaged with Europe.

U.S. President Barack Obama had urged Britain not to quit Europe. The two countries’ Special Relationship — a phrase first used by the late prime minister Winston Churchill — had enabled the United States to have a greater voice in the EU through Britain.

The Obama administration has reacted with concern to the Brexit referendum and what it will mean for relations with London and with the EU.

“The United Kingdom and the European Union will remain indispensable partners of the United States even as they begin negotiating their ongoing relationship,” President Obama said in a statement Friday.

In London, Kerry will find a political situation in flux. Having lost his mandate in the referendum, Prime Minister David Cameron announced his resignation Friday and has clearly indicated he will not be the one to negotiate the formalities of Britain’s separation from the EU.

Cameron is to name a negotiating team to begin the process, but the formal notification that would start the disengagement may not come until after October when a new prime minister, of Cameron’s Conservative party, is expected to take over.

Brexit has also sent Britain’s Labour party into turmoil, with calls growing for party leader Jeremy Corbyn to resign. Party members accuse him of having failed to galvanize support within Labour to defeat Brexit.

Meanwhile, founding member states of the European Union are pressing for Britain to leave quickly.

This week will see much shuttle diplomacy as officials work to contain Brexit fallout.

Before London, Kerry will meet with EU foreign policy chief Federica Mogherini Monday at EU headquarters in Brussels. The top U.S. diplomat was in Rome Sunday to discuss the stalled Middle East peace process.

In London, Kerry will begin a process of shoring up a relationship in a drastically new landscape, and with a group that was deeply offended by what it viewed as President Obama’s meddling in Britain’s internal affairs.

Some British voters who cited sovereignty as their main reason for voting to leave the EU said Obama helped them make the decision when he visited Britain in April and warned them against voting for a Brexit, telling them Britain would go to the back of the queue on trade deals if they voted “out.”

“We wanted our democracy back. We wanted to take control control of our country,” said London voter Trevor Bayley. “I think your president unwittingly had a part to play in that. He tried to shore up the establishment vote, and people didn’t like to be told what to do.”

On Tuesday, Cameron travels to Brussels where the EU’s other 27 member states will for the first time meet without Britain.